Conventionally, audio encoding and decoding methods, which are international standards by ISO/IEC, have been widely known, for example, so-called MPEG methods. Presently, there is an encoding method known as ISO/IEC13818-7, called MPEG-Advanced Audio Coding (MPEG-2AAC), which has wide applications and aims at encoding high-quality audio signals at low bit rates.
According to this AAC, when audio signals detected by multiple channels are to be encoded, a correlation between the channels is obtained using a system called a Mid Side (MS) stereo or an intensity stereo, and the correlation is considered in compressing the audio data to improve coding efficiency.
In the MS stereo, stereo signals are represented by a sum signal and a difference signal, each of which is allocated with a different coding amount. On the other hand, in the intensity stereo, each frequency band of signals from plural channels is segmented into multiple sub-bands, and a level difference and a phase difference (the phase difference has two stages of an in-phase and an anti-phase) in signals between the channels are encoded regarding each of the sub-bands.
A number of standards extended from this AAC are currently being developed. In the development, a coding technology using information called spatial cue information or binaural cue information is planned to be introduced. One example of such a coding technology is a parametric stereo system according to a MPEG-4 Audio (non-patent reference 1) which is an international standard by the ISO. Other examples are technologies disclosed in patent references 1 and 2.    [Patent Reference 1] U.S. Patent Application Publication No. 2003/0035553 entitled “Backwards-compatible Perceptual Coding of Spatial Cues”    [Patent Reference 2] U.S. Patent Application Publication No. 2003/0219130 entitled “Coherence-based Audio Coding and Synthesis”    [Non-Patent Reference 1] ISO/IEC 14496-3:2001 AMD2 “Parametric Coding for High Quality Audio”